How to get realistic dirt on metal?
Thanks for the responses everyone. I realize I was asking several things at once. Here's what I figured out:


The sun lamp was too dim. That's why the results were not comparable with the HDRI.
I had to really turn up the the "Strength" and "Distance" settings on the bump node to get some effect. And turning down the roughness setting allows the "Dirt" to look like it's sitting on top of the metal.


Unfortunately, the effect looks more like metal than dirt, but the questions I was asking in this thread are answered.



Edit: I found that inverting the Bump node, gave more of the look I was going for: